1. Field of the Invention
The invention relates to ketal compounds that are useful as part of a coating composition for positive-working, lithographic printing plates, which are imagewise exposed to near-infrared radiation and then developed. More particularly, the invention relates to a particular class of ketals, namely, 1,1-di[(alkylphenoxy)ethoxy]cyclohexanes. These ketals comprise hydrophilic groups and lipophilic groups. In general, the ratio of the molecular mass of the hydrophilic groups relative to the molecular mass of the lipophilic groups determines the solubility of these ketals in different solvents. These ketals may be decomposed by acids to form surfactants, which can have solubility characteristics that are different than those of the starting ketal.
2. Description of the Related Art
It is known in the art that alcohols will react with the carbonyl group of aldehydes in the presence of an acid catalyst to yield acetals. These reactions are reversible. Therefore, the reaction is generally carried out by allowing the carbonyl group to react with an excess amount of the alcohol and by removing the water byproduct. Furthermore, it is also known in the art that alcohols are generally more difficult to react with the carbonyl group of ketones, in particular, cyclohexanone, to yield ketals, in this case, 1,1-dialkoxycyclohexane.
Simple 1,1-dialkoxycyclohexanes can be prepared by other methods, such as, by reacting a simple alcohol with cyclohexanone in the gas phase. This is described, using methanol, by K. B. Wiberg, K. M. Morgan, and H. Maltz in Thermochemistry of Carbonyl Reactions. 6. A study of hydration equibria, J. Am. Chem. Soc., 1994, 116, 11067-11077. Another method for making 1,1-dimethoxycyclohexane is by reacting methanol with cyclohexanone in the presence of a solid acid catalysts, as described by B. Thomas, S. Prathapan, and S. Sugunan in Synthesis of dimethyl acetal of ketones: design of solid acid catalysts for one-pot acetalization reaction, Microporous and Mesoporous Materials, 2005, 80, 65-72.
Other methods have been devised to produce simple 1,1-dialkoxycyclohexanes. For example, they can also be prepared by reacting cyclohexanone with a trialkoxymethane, such as trimethoxymethane, triethoxymethane, or triisopropoxymethane. For example, trimethoxymethane may be added to the carbonyl group of the cyclohexanone in the presence of an acid catalyst to yield the ketal and byproduct methyl formate. Such a reaction is described in U.S. Pat. No. 5,399,778.
Non-simple 1,1-dialkoxycyclohexanes are very difficult to make by reacting cyclohexanone with alcohols having long straight chains, branched chains, or aromatic groups. For example, 1,1-di(cyclohexyloxy)cyclohexane was made with a yield of 29% by reacting cyclohexanol with cyclohexanone, as described in U.S. Pat. No. 3,072,727. Non-simple 1,1-dialkoxycyclohexanes are typically made by transketalization, namely, by mixing and heating such alcohols with an already formed simple 1,1-dialkyoxycyclohexane, namely, with methoxy, ethoxy, or isopropoxy groups. For example, 2-phenoxyethanol can be mixed with 1,1-dimethoxycyclohexane to form 1,1-di(2-phenoxyethoxy)cyclohexane. This reaction is described in U.S. Pat. No. 6,165,676 and U.S. Pat. No. 6,391,512.
There is great commercial interest in lithographic printing plates having coatings that are digitally imageable using near-infrared laser exposure. With positive-acting plates, once the coating is imagewise exposed to infrared radiation, the exposed areas are easily removed, while the unexposed areas remain as the image. Positive-working, imageable compositions containing novolak or other phenolic polymeric binders and diazoquinone imaging components have been prevalent in the lithographic printing plate and photoresist industries for many years. Imageable compositions based on alkali-soluble phenolic resins with various dissolution inhibitors and infrared radiation absorbing compounds are also well known. The dissolution inhibitors are believed to prevent dissolution by hydrogen bonding of the normally alkali-soluble phenolic resins prior to imaging. During actinic exposure, it is generally believed that the mechanism of image formation occurs by an increase in alkaline solubility due to a disruption or breaking of the hydrogen bonds.
The use of dissolution inhibitor compounds having acid-cleavable C—O—C groups in positive-working printing plates is also well known. Representative of such compounds are 2-tetrahydropyranyl ethers described in U.S. Pat. No. 3,779,778; ortho-carboxylic acid esters described in U.S. Pat. No. 4,101,323; polyacetals described in U.S. Pat. No. 4,247,611; and ketals described in U.S. Pat. No. 6,165,676. These compounds prevent dissolution of normally alkali-soluble phenolic resins in alkaline developer solutions.
Moreover, these dissolution inhibitors are generally mixed with photolytic acid-generating compounds in the photosensitive layers of the printing plates. Upon imagewise exposure of the layers to actinic radiation, an acid is released from the photolytic acid-generating compound that then catalyzes the decomposition of the dissolution inhibitors in the exposed regions. When this occurs, the exposed region can then be selectively dissolved in an aqueous-based, alkaline developer.
Simple ketals of 1,1-dialkoxycyclohexane are generally poor dissolution inhibitors, due to the lack of hydrogen bonding sites. Examples of simple ketals include 1,1-dimethoxycyclohexane (Mw 144), 1,1-diethoxycyclohexane (Mw 172), and 1-1-di(1-methoxyethoxy)cyclohexane (Mw 200).
A particular class of ketal dissolution inhibitors of the class 1,1-di[(alkylphenoxy)ethoxy]cyclohexanes having the general formula[(R′)rC6H(5−r)O(CH2CH2O)m][(R″)sC6H(5−s)O(CH2CH2O)n]C6H10 wherein m and n independently are an integer of from 1 to 5; r and s independently are an integer of from 1 to 3; and R′ and R″ independently are a hydrogen atom or an alkyl group having 1 to 5 carbon atoms are described in U.S. Pat. No. 6,165,676; U.S. Pat. No. 6,391,512; U.S. Pub. No. 2007/0172758; and U.S. Pub. No. 2009/0075201. The alkyl groups of R′ and R″ may be straight or branched, and include methyl, ethyl, propyl, butyl, and pentyl groups. The specific example of such ketals in the above-mentioned patents and patent applications is the ketal wherein m and n are 1; and R′ and R″ are hydrogen atoms. The formula for this specific ketal is [C6H5O(CH2CH2O)][C6H5O(CH2CH2O)]C6H10. This 1,1-di(2-phenoxyethoxy)cyclohexane is water insoluble. Strong acids will decompose this ketal into cyclohexanone and 2-phenoxyethanol, which has a low solubility of 27 g/l in water. It is desired that the compounds in the photosensitive layer after exposure to near-infrared light be soluble in an aqueous developer. Due to their water-insoluble characteristics, neither this specific ketal nor its acid-decomposed part is suitable for an aqueous developer. Furthermore, due to the relatively low molecular weight of 356 for this specific ketal, lithographic coatings with this ketal tend to have lower printing durability in the image areas of the printing plate. In addition, due to the lack of lipophilic alkyl groups on this specific ketal, coatings with this ketal tend to not accept printing inks well.